The demand for improved efficiency of reheat furnaces has resulted in the use of burners requiring two separate air supplies or the intermixing of large and small burners across the width of a furnace. In either case, this requires two separate air supply headers. Due to the very large sizes involved, the cost is very high and this doubles the normal quantity of pipe generally used in this area resulting in an additional restriction in an area that is normally conjested, thus increasing the maintenance cost of the furnace system.
While not limited thereto, the present invention is particularly adapted for use with industrial heating furnaces such as reheating furnaces used to heat steel slabs to the hot-working temperature. In such furnaces, the steel slabs are brought up to the hot-working temperature as they are conveyed through a furnace by means of a pusher, walking beam or other device. The reheat furnace temperature through the length is not uniform, being colder at the charge end and hot where discharged. This furnace can have one zone of temperature control or have multiple zones of independent temperature control. The steel, as it progresses through the furnace, gets hotter and reaches the maximum temperature and best uniformity just prior to being discharged from the furnace. Heating the steel faster causes more fuel to be used and increases the formation of scale on the steel surface. To prevent or minimize the deterrent effects, the combustion system must have the ability to operate over a wide range of inputs with minimum amount of excess air, particularly in the case where there is a breakdown in the rolling mill complex and steel flow through the reheat furnace must stop. Under this condition, it is desirable to keep steel temperature at its present level or reduce the temperature to some lower value. This is the condition that requires the combustion system to operate at a lower than normal input. In fact, during long delays the fuel input is only needed to overcome the heat loss through the furnace walls, hearth and roof. This input can be 5% to 10% of the normal fuel input for this zone. The normal burner can operate efficiently at 20% flow but below that requires additional air which becomes detrimental to steel surface conditions (increased scale) and also decreases combustion efficiency. The use of large and small burners intermixed across the width of the furnace allows the larger burner to be shut-off and the smaller burners to operate at an input that is still efficient. Both burners require an air supply header.